In the source material, Stein combines with student Ronnie Raymond to form the matter-transmuting hero known as Firestorm, with Raymond in control of the hero’s body while the professor guides him as a voice in his mind, offering advise and scientific help. Raymond was cast months ago, with Robbie Amell, a CW vet and cousin of “Arrow” star Stephen Amell, set to bring the character to life.

With the addition of Dr. Stein, “The Flash” bolsters its roster of Firestorm-related characters, as villains Plastique, Multiplex and Caitlin Snow — the civilian identity of Killer Frost — all originated in the character’s comic adventures.